1. Field of the Invention
The present invention relates to air filters, and more particularly to the air filter having improved capture efficiency of micro-organisms contained in the air; and even more particularly, the present invention relates to an air filter having combined electrically enhanced filtration and surface discharge (plasma) or non-discharge sterilization for destruction of the captured micro-organisms.
2. Prior Art
The problem of purification and filtration of indoor air is an important one and cannot be over-estimated. Tuberculosis, legionella, sinusitis, allergies, bronchitis, asthma, and other health problems can be caused to the large extent by the indoor air pollution. Therefore, air filtration systems providing an adequate particle removal efficiency are constantly needed for purification of the indoor air.
Numerous air filters with electronically enhanced capturing capability have been described in the literature and are available in the commercial marketplace. In some of these systems, the improvement of a filter's capture efficiency is achieved through the application of electrostatic fields to a filter. For example, a high efficiency electronic air filter is disclosed in U.S. Pat. No. 5,573,577 in which pads of dielectric fibers are sandwiched between electrically charged ionizing elements, and grounded screens. The ionizing elements charge the dust particles passing through the filter and at the same time, polarize the fibrous filter pads. In this way, the charged particles are attracted and collected on the fibrous pads with improved efficiency.
As disclosed in U.S. Pat. No. 5,405,434, an electrostatic filter for purifying air in an EVAC system includes a pair of conductive filaments insulated from one another and disposed close together in a substantially parallel side-by-side relationship. Circuitry is provided for applying an electrical potential difference between two conductors. The strong electric fields cause the wire sets to attract fine airborne particulate matter in the vicinity of the filter mesh so that the mesh retains dirt, atmospheric ions, and other very fine particles. Such particles include pollen and bacteria borne by the air stream passing through the mesh which are removed from the air.
U.S. Pat. No. 5,593,476 describes a high efficiency air filtration apparatus utilizing a fibrous filter medium that is polarized by a high potential difference which exists between a pair of electrodes. The electrodes include an insulated electrode and an uninsulated electrode. A corona precharger is positioned upstream of the electrodes and filter. The corona precharger applies a charge to particles which are removed from the air flow system as they accumulate on the filter surfaces proximal to the insulated electrode.
Although filters are good candidates for removing sub-micron airborne particles (0.3 micrometer diameter particles are captured with efficiency greater than 99%), their capture efficiency however decreases rapidly for particle diameters below 0.3 microns. This is a major disadvantage of electrically enhanced high efficiency particulate air filters since these filters fail to effectively purify the indoor air from airborne micro-organisms hazardous to health. The filtration of bacteria and viruses from indoor air is hindered by two characteristics of the organism which are the extremely small size of the organisms and the ability of the organisms to propagate. The typical diameter of bacteria is a few micrometers, however, viruses can be 1/100th of this diameter. Therefore, it is not only difficult to capture airborne pathogens on the filter material due to their small dimension, but also the organisms that are captured by the filter may propagate on the filter surface and migrate through the filter. These combined factors necessitate frequent filter changes. The control of airborne pathogens in an indoor environment is especially acute due to the fact that a major, if not predominant source of airborne micro-organisms results from entrainment of colonies that have grown on the filter.
In external environments such as outdoor air, the micro-organisms die as a result of sunlight, temperature extremes, and dehydration. However, for indoor conditions, the range of temperatures is relatively narrow and the air is shielded from direct sunlight. Further indoor air may be humidified, thus aiding in propagation of the organisms. Relative to outdoor air, the quality of indoor air can be 20-70 times worse. An example of the seriousness of biological contamination in indoor air is Legionnaires disease. The legionella bacteria were first discovered in 1976 as a result of the Legionnaires disease outbreak in Philadelphia that caused 200 cases of this disease. Legionella was found to be the cause of a similar outbreak the previous year at the same hotel as well as a series of mysterious epidemics going back 50 years.
Another example, tuberculosis, is spread via the air through inhalation. Microbacterium tuberculosis is carried in airborne particles known as droplet nuclei that are generated when persons with pulmonary or laryngeal tuberculosis sneeze, cough, speak, expectorate, or exhale. The droplet nuclei are so small (1-5 micrometers) that they can be suspended indefinitely in the air and be spread throughout a facility by an HVAC system. The probability that a susceptible person becomes infected with microbacterium tuberculosis depends primarily upon the concentration of infectious droplet nuclei in the air and the exposure duration. Unlike other airborne diseases, which require large aerosolized colonies of bacteria to produce an infection, one tuberculosis bacillus is enough to infect humans.
None of the known filter systems, even those with electrically enhanced capture efficiency appear to prevent micro-organisms propagation and entrainment into the indoor air from the filter.
It is, therefore, clear that the problem of airborne pathogens in indoor environments has not been solved by the electrically enhanced filters which are known to those skilled in the art. Therefore, a new, improved air filter is needed which would not only has an increased capturing efficiency, but also provides a biocontaminant control of the indoor environment by impeding propagation of the pathogens on the filter surface and preventing re-entrainment of the organisms into the air from the filter.